The present invention relates to a solid electrolytic capacitor, and more particularly to a solid electrolytic capacitor utilizing a conductive polymer as a solid electrolyte.
A solid electrolytic capacitor has a configuration constituted by forming an anode with a valve-action metal such as tantalum, niobium or aluminum, oxidizing a surface of the anode to form a dielectric member, forming thereon a closely contacting layer of manganese dioxide or a conductive polymer, and forming on that layer a graphite layer or a silver paste layer as a cathode.
The manganese dioxide or the conductive polymer constitutes a solid electrolyte, and particularly the conductive polymer is being employed frequently in recent years, as it has a high conductivity and can reduce an equivalent series resistance (ESR) of a capacitor.
The solid electrolytic capacitor utilizing a conductive polymer as a solid electrolyte is disclosed for example in Japanese Patent Application Laid-open No. 5-275290 (which will be hereinafter referred to as related art 1).
In a solid electrolytic capacitor based on the related art 1, an aluminum foil subjected to a surface enlargement by etching, which will be referred to as an etched aluminum foil, is divided by an epoxy resin member, into a first area for connecting an external anode terminal, starting from an end in the longitudinal direction, and a second area for realizing a capacity. In the first area which constitutes a major portion of a side of the aluminum foil, an aluminum oxide film is formed which is obtained by anodizing the aluminum foil. A polypyrrole layer is formed by a conductive polymer and is provided on such aluminum oxide film. On the polypyrrole layer, a graphite layer and a silver paste layer are formed in succession and in closely contacted manner. A basic structure of a capacitor, which will be referred to as a solid electrolytic capacitor element, is formed by the aluminum foil, the aluminum oxide film, the polypyrrole layer, the graphite layer and the silver paste. The anode is constituted by the aluminum foil, the dielectric member is constituted by the aluminum oxide film, and the cathode is constituted by the polypyrrole layer, the graphite layer and the silver paste. For electrical connection with the exterior, an external anode terminal and an external cathode terminal are mounted on the capacitor element, and an external package is provided for sealing the element and shaping the external form of the capacitor.
The external anode terminal and the external cathode terminal are formed by a base member formed by a copper-based or iron-based metal and surfacially plated with a metal for enabling soldering. The external anode terminal is adjoined by an electric welding or an ultrasonic welding to the aluminum foil constituting the anode, while the external cathode terminal is electrically fixed, for example by a conductive adhesive material, to an outermost layer of the cathode.
The conductive polymer employed in the prior solid electrolytic capacitor is associated with a drawback, though initially having a high conductivity, of losing the conductivity by being oxidized in an oxygen-containing atmosphere of a high temperature. Therefore, in case the interception of oxygen is insufficient in the cathode, ESR will be deteriorated when the solid electrolytic capacitor is exposed to a high temperature over a long period. In the related solid electrolytic capacitor, the cathode is constituted by forming a graphite layer and a silver paste layer on a conductive polymer layer. The graphite layer and the silver paste layer cannot be formed densely and is therefore unable to sufficiently prevent intrusion of oxygen.
Also in the prior solid electrolytic capacitor, the external cathode terminal has a solder plating or a tin plating on the surface of a base metal, in order to enable soldering thereby facilitating the mounting. Such plating certainly facilitates soldering, but is associated with a drawback of being partially fused in a surface adhered to the cathode of the capacitor element, thereby causing a peeling of the external cathode terminal and the conductive adhesive and increasing a contact resistance with the cathode electrode.